The objective is to study the mechanical factors influencing the pattern of pulmonary capillary blood flow in animals so measurements in man by the N2O-plethysmograph technique may be better used to assess derangements of the left heart and pulmonary venous system and right heart and pulmonary arterial system. It is possible by using retrograde Macklem-Mead catheters in the small arteries and veins to measure pressures on either side of the pulmonary capillaries and derive the driving pressure for the pulmonary capillary flow pulse. Similarly the driving pressure for venous flow is given by the difference between upstream venous and left atrial pressures. There are several pulmonary venous flow pulses in each heart cycle, some due to left atrial activity. However it is possible to identify a venous flow pulse transmitted from the pulmonary artery if left atrial pressure is temporarily removed from the downstream end of the vein. This is done with a sleeve-valve venous cannula which can shunt the venous blood to a reservoir or the left atrium. The effect of varying downstream pressure, alveolar pressure and interstitial edema on the transmission and damping of the antegrade and retrograde pulses can then be measured.